Could you please tell me if there are any special seo rules to keep in mind when developing or commissioning an ecommerce site?
It’s hard to answer that question because every ecommerce site is different and presents its own challenges.
For example, if it is a large e-commerce site, it may be database driven, which means that product pages may contain session ids or multiple parameters that might confuse search engines. That would require a server-side solution of some kind.
It might offer thousands of products, meaning keyword research is a huge job and optimizing individual pages is extremely time consuming. That might require a script to integrate a meta tag / title tag template on every page.
It might be in a very competitive industry, meaning SEO of pages may not have much of an impact unless the site is hugely popular with very strong Google PR and backlinks.
I would say the most important thing you can do before SEOing an ecommerce site is RESEARCH. Your SEO Requirements Document will be crucial here, as will a very good dig into the site and the client company. To learn more about what should be included in your SEO Requirements Document, see my article: Before Launching Your SEO Campaign.
Learn as much as you can about the company, their customers, their goals and their target markets before you start any SEO activity.